This project aims to understand the function of sex differences in neural control of social behavior by focusing on the higher level of vasopressin expression in male versus female brains. This is the most consistently found sex difference among vertebrates, and the best understood in terms of function in adult animals. This grant will study whether the sex difference in vasopressin innervation contributes to sex differences in the display and control of social behavior. We will take advantage of new powerful genetic approaches that allows us to target vasopressin cells specifically. The first aim is to test whether removal of the sexually dimorphic vasopressin cells in the bed nucleus of the stria terminalis and medial amygdaloid nucleus affects social behavior differently in males and females. The second aim is to test whether inhibition or excitation of these cells alters social behavior differently in males and females. The third aim is to identify the in- and outputs of these cells, to begin to understand the neural circuitry via which vasopressin affects social behavior differently in males and females. Understanding how the brain controls social behavior differently in males and females is important as many behavioral disorders show striking sex differences in morbidity. Sexual differentiation of the brain likely contributes to these differences. Although recently major advances have been made in understanding the neural basis of social behavior in adulthood, how such behavior is controlled differently in males and females is, by and large, unknown. This grant will address these issues.